Metal based porcelain dental restorations such as crowns and bridges comprise a metal framework called a "coping", which is covered by several layers of porcelain to simulate the appearance of natural teeth. The porcelain is applied in a plurality of layers, the first of which is called the "opaque porcelain layer", the purpose of which is to hide the metal framework. The second layer is called the "body porcelain layer". The body porcelain layer exhibits translucence to a degree similar to that of the dentine layer of natural dentition. In addition, there is often a third layer, on top of the body porcelain layer, called the "incisal porcelain layer". The incisal porcelain layer approaches the translucency of the enamel layer of natural dentition. The outer surface is either glazed with a very thin transparent layer, or it is baked to a high gloss.
Color is imparted to a metal-based porcelain dental restoration by coloring the opaque and body porcelain layers. Naturally, it is a major objective in the production of dental restorations to make the restoration resemble as closely as possible the patient's natural teeth.
Recently, all-ceramic dental restorations have been introduced commercially. These restorations replace the metal coping with a ceramic base, and because the metal base is eliminated, they can be made to more closely resemble natural dentition. But even with an all-ceramic restoration, there is obviously still a need to match the color of the patient's natural teeth. One way to color an all-ceramic restoration is to color the ceramic base by any of several techniques (e.g., the color may be incorporated in the base material itself, the base may be stained with a porcelain stain, or the base may be covered with a layer of the desired color called "dentino porcelain"--dentino porcelain is less translucent than body porcelain), and to then add a layer of body porcelain. The body porcelain may be followed by an incisal layer and a glaze, as is the case with metal-based restorations.
Whether the porcelain restoration has a metal base or is all ceramic, its apparent color is influenced by the color of the body porcelain layer and by the color of the layer just beneath the body porcelain. The incisal porcelain and glaze layers contribute little, if anything, to the perceived color of the restoration because they are quite translucent or transparent, and are, at most, only slightly colored. Since natural teeth have translucent layers, i.e., enamel and dentine, the restoration must have translucent layers on its surface to match as closely as possible the appearance of natural teeth. However, the translucency of the body porcelain layer complicates the task of matching the color of natural teeth. The thickness of the body porcelain varies from a rather thick layer at the incisal tip to a thin layer at the gingival end of the restoration. Thus, it is normal for the body porcelain layer thickness to vary from about 1/2 to 11/2 millimeters. Because of this variation in thickness, light penetrates the body porcelain layer to different depths before it is reflected back to the observer, and unless the layer just beneath is exactly the same color as the body porcelain, the apparent color of the restoration will vary over its surface with the thickness of the body porcelain. The reason for this is the following:
Whereas the color of an opaque object is determined by the amount of visible light reflected (from the surface of the object) to the observer by the illuminating light, the perceived color of a porcelain dental restoration is mainly the result of the diffuse reflectance from the translucent body porcelain layer covering an underlying more opaque layer. Perceived color in such a case will be a combination of the reflected color of the translucent layer plus the color reflected from the underlying layer. When the translucent layer varies in thickness, the amount of color contribution from the underlying layer will vary inversely with the thickness of the translucent layer. Therefore, unless the translucent layer and the underlying layer are exactly the same color, the perceived color of the restoration will be dependent upon the thickness of the body porcelain layer.
For the reasons discussed above, it is clear that it is desirable to provide porcelain dental restorations in which the body porcelain matches the color of the layer beneath. Heretofore, however, as far as is known to the inventors herein, there has been no commercial dental porcelain restoration kits in which the body porcelain material, when fired, matched exactly the color of the fired ceramic material of the layer just beneath. In order to compensate for this, the dental technician has often had to apply porcelain stains to different portions of the restoration to prevent the perceived color from varying over its surface to an undesirable degree. This was not only a time consuming task, but also the results were quite dependent upon the skill of the technician.
This invention provides a means for solving the problem discussed above. According to the invention, porcelain can be produced having a predetermined color to match the color of an underlying, more opaque, layer such that the eye cannot discern any difference in color in a composite comprising a layer of the translucent porcelain overlying said underlying layer, even though the translucent porcelain layer varies in thickness.